- Intermittent Diagnosis

SYMPTOM DESCRIPTION:
Problems may or may not turn "ON" the SERVICE ENGINE SOON light, or store a code.


CAUTION: DO NOT use the TROUBLE CODE CHARTS in DIAGNOSTIC CHARTS for intermittent problems. The fault must be present to locate the problem. If a fault is intermittent, use of trouble code charts may result in replacement of good parts.



FAULTY ELECTRICAL CONNECTIONS OR WIRING

Most intermittent problems are caused by faulty electrical connections or wiring. Perform a careful visual check of suspected circuits for:
^ Poor mating of the connector halves, or terminals not fully seated in the
connector body (backed out).
^ Improperly formed or damaged terminals. All connector terminals in
problem circuit should be carefully reformed or replaced to insure proper
contact tension.
^ Poor terminal to wire connection. This requires removing the terminal
from the connector body to check.



ROAD TEST

If a visual/physical check does not find the cause of the problem, the vehicle can be driven with a voltmeter connected to a suspected circuit or a SCAN tool may be used. An abnormal voltage or "Scan" reading, when the problem occurs, indicates the problem may be in that circuit. If the wiring and connectors checkout OK, and a trouble code was stored for a circuit having a sensor, except for Codes 44 and 45, replace sensor.

NOTE: It would be advantageous to make note of any codes stored and clear the ECM/PCM memory prior to road testing. This may eliminate any false codes not related to the complaint.

NOTE: A SCAN tool is required to perform test using TROUBLE CODE CHARTS.



INTERMITTENT "SERVICE ENGINE SOON (SES) LIGHT"

An intermittent "SES" light, and No Diagnostic Codes, could be caused by:
^ Electrical system interference caused by a defective relay, ECM/PCM driven
solenoid, or switch. These components can cause a sharp electrical surge.
Normally the problem will occur when the faulty component is operated.
^ Improper installation of electrical devices, such as lights, 2-way radios,
electric motors....etc.
^ EST wires should be routed away from spark plug wires, ignition system
components and generator. Wire for CKT 453 from ECM/PCM to ignition
system should be a good ground.
^ Ignition secondary shorted to ground.
^ CKT 419 (SERVICE ENGINE SOON light) or CKT 451 (diagnostic "test"
terminal) intermittently shorted to ground.
^ ECM/PCM grounds.



LOSS OF DIAGNOSTIC CODE MEMORY

To check, disconnect TPS and idle engine until SERVICE ENGINE SOON light comes "ON," Code 22 should be stored, and kept in memory when the ignition is turned "OFF" for at least 10 seconds. If not, the ECM/PCM is faulty.